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The future of quantum roulette noise thermometry.

C H Webster

Published Jan 1, 2006• 1 citation
OpenAlex

Abstract

The Quantum Roulette Noise Thermometer (QRNT) is a unique type of primary thermometer. Rather than probing a classical thermodynamic variable whose value depends on the average motion of an ensemble of microscopic particles, it makes a series of consecutive measurements of a quantum variable, the flux trapped within a superconducting ring. From the statistical distribution of flux measurements the temperature can be obtained. In principle, the device could operate over a wide range of temperatures, with the upper limit dependent on the superconducting ma- terial chosen for the ring. Its accuracy would be limited by statistical sample size. The QRNT directly addresses the primary objectives of the UK National Measurement System (NMS) Quantum Programme: an NMS based wholly on quantum processes, fundamental constants and the use of portable quantum standards with full primary accuracy. This report reviews preliminary research and assesses the future benefit of QRNT devices to thermal metrology.

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The Quantum Roulette Noise Thermometer (QRNT) is a unique type of primary thermometer. Rather than probing a classical thermodynamic variable whose value depends on the average motion of an ensemble of microscopic particles, it makes a series of consecutive measurements of a quantum variable, the flux trapped within a superconducting ring. From the statistical distribution of flux measurements the temperature can be obtained. In principle, the device could operate over a wide range of temperatures, with the upper limit dependent on the superconducting ma-
terial chosen for the ring. Its accuracy would be limited by statistical sample size. The QRNT directly addresses the primary objectives of the UK National Measurement System (NMS) Quantum Programme: an NMS based wholly on quantum processes, fundamental constants and the use of portable quantum standards with full primary accuracy. This report reviews preliminary research and assesses the future benefit of QRNT devices to thermal metrology.
A novel form of primary thermometer called the quantum roulette noise thermometer (QRNT), based on flux quantization in a high temperature superconducting (HTS) ring, interrupted by a Josephson junction, has been proposed. The basis of the QRNT is that the flux state of the ring is allowed to evolve freely, and is repeatedly interrogated to establish a probability distribution from which the temperature of the device is derived. Unlike other superconductivity-based noise thermometers, this device is essentially dissipationless, requires no input or output connections, and may be remotely interrogated. The QRNT has the potential to provide high accuracy with short measuring times. Prototype QRNT devices based on thin HTS films have been fabricated and results of initial experimental measurements are presented. Parameters of the experimental devices are compared with predictions of the theoretical model.
We describe a cryogenic primary thermometer called the Quantum Roulette Noise Thermometer (QRNT), which is based on flux quantisation in a HTS superconducting ring. The basis of the QRNT is that the flux state of a double junction SQUID ring (in thermal contact with a heat bath whose temperature is to be measured) is allowed to evolve freely. The flux state is repeatedly interrogated to establish a probability distribution from which the absolute temperature may be derived. The QRNT is in principle dissipationless and has the potential to provide high accuracy with short measuring time. The QRNT prototype device which employs a YBCO thin film on an MgO bi-crystal substrate has been designed and fabricated. The superconducting ring of overall dimensions 6 mm/spl times/8 mm is broken by two grain boundary Josephson junctions of size 5 /spl mu/m/spl times/3 /spl mu/m. The state of the junctions is switched between superconducting and resistive by means of small currents applied to a superconducting strip line located near the junctions. A dc SQUID, which is used to read out the flux state of the QRNT ring, is operated at 77 K while the QRNT device can be set at any desired temperature above 10 K. Ongoing experimental measurements are described with reference to the expected theoretical predictions.
A novel form of primary thermometer called the Quantum Roulette Noise Thermometer (QRNT) has been fabricated and is presently undergoing feasibility studies. This device utilises flux quantisation in a high temperature superconducting ring interrupted by two closely spaced, parallel, Josephson junctions. Several devices have been fabricated and different methods investigated for measuring the statistical distribution of trapped flux states. We report the results of these endeavours with emphasis on a method for suppressing I/sub c/ by applying an r.f. field to the junctions.
We have been developing a Johnson noise thermometer using an integrated quantum voltage noise source (IQVNS) as a reference signal source. IQVNS is a superconducting integrated circuit that generates pseudo random signals. The power spectral density of output signal of IQVNS can be described with quantum accuracy by fundamental physical constants e and h, the clock frequency of the circuit, and a numerical coefficient. In the previous version of IQVNS the numerical coefficient was fixed by the circuit design. However, to cope with the measurement of thermodynamic temperature in a wide temperature range, it is desirable to make the output of IQVNS variable. In this study, we improved part of the design of the device to be able to change the power spectral density of the output signal and confirmed by measurement that the output signal can be variable as designed.
The rapidly developing quantum technologies and thermodynamics have put forward a requirement to precisely control and measure the temperature of microscopic matter at the quantum level. Many quantum thermometry schemes have been proposed. However, precisely measuring low temperature is still challenging because the obtained sensing errors generally tend to diverge with decreasing temperature. Using a continuous-variable system as a thermometer, we propose non-Markovian quantum thermometry to measure the temperature of a quantum reservoir. A mechanism to make the sensing error $\ensuremath{\delta}T$ scale with the temperature TTT as the Landau bound $\ensuremath{\delta}T\ensuremath{\simeq}T$ in the full-temperature regime is discovered. Our analysis reveals that it is the quantum criticality of the total thermometer-reservoir system that causes this enhanced sensitivity. Efficiently avoiding the error-divergence problem, our result gives an efficient way to precisely measure the low temperature of quantum systems.